SUMMARYTo study the possibility of secondary hyperparathyroidism in patients with Paget's disease treated with calcitonin over a long period, we investigated parathyroid function in eleven patients treated with salmon calcitonin (CT) for up to 23 months and in one patient treated with human CT. Basal plasma calcium, immuno‐reactive parathyroid hormone (PTH) and serum phosphorus levels were all in the normal range before and after the treatment and did not change significantly during the treatment. In all patients, an acute fall in calcium and a rise in PTH occurred after the initial injection of CT. The rise in PTH was proportional to the fall in calcium and similar to that observed in normal subjects. After treatment, the rise in plasma PTH remained the same in relation to the fall in calcium, indicating that the responsiveness of the parathyroid glands to the hypocalcaemic challenge was unaltered, and that secondary hyperparathyroidism was not present in these patient